Study on Reloading Stress Relaxation Behaviour for High Temperature Bolted Steel

Designing and maintenance of mechanical elements is very important to clarify the effect of retightening (reloading) on stress relaxation behaviour of high temperature bolts. Experiments of stress relaxation simulates the real engineering problem of the long-term loosening of tightened bolts and other fasteners. Stress relaxation curves for a bolted steel 25 NiCrMo 8 are analyzed at three levels of test temperature (400, 450 and 500°C). The effect of the specified relaxation stresses on the reloading stresses behaviour is studied using five levels of the specified relaxation stress ratio (0.9, 0.8, 0.7, 0.6 and 0.5). Discussions are made on the relation between the testing time at each cycle and the number of loading with the variable of the specified relaxation stress ratio and the test temperature. The plastic strains accumulated during the course of the testing are analyzed and calculated by using empirical equation. Moreover, the apparent activation energy of relaxation behaviour which is determined as a relation of the testing time and of the specified relaxation stress ratio can be calculated.